You can get a pretty good idea for how sales are going by looking at the owners lists on Pinside. You have to factor in how long the game has been out and then a comparison can be made with other titles.
Sales are going to be down because there are a lot of choices for buyers. Also, with MMr having just been announced not very long ago, surely that has taken a lot of the money away that Stern would have had a chance to get a good portion. Even if a game does not sell well as compared to other titles that does not mean it is not a good game.